Transparent glass ceramic with small thermal expansion

ABSTRACT

Colorless, transparent glass ceramic with a linear thermal expansion which is smaller than 15 × 70 -   7  /°C in the temperature range from 20° to 300°C, produced from a glass, which has been melted and shaped and thereafter subjected to a heat treatment for the controlled crystallization of innumerable, irregularly oriented, minute crystals of the structural type of β-quartz, characterized in that it has, to at least 85% by weight, the following composition based on oxides: 
     
         ______________________________________                                    
 
    
     SiO 2    45 to    68%     by weight                                    
Al 2  O 3                                                           
            15 to    30%     by weight                                    
P 2  O 5                                                            
            0 to     10%     by weight                                    
Li 2  O  2 to     6%      by weight                                    
MgO         0 to     3%      by weight                                    
ZnO         0 to     8%      by weight                                    
ZrO 2    2 to     7%      by weight                                    
Ta 2  O 5                                                           
            1 to     7%      by weight                                    
______________________________________                                    
 
     The contents of Li 2  O, MgO, ZnO and P 2  O 5  being such that the existing Al 2  O 3  content, to within a residue or a deficit of Al 2  O 3  of up to about 4% by weight, is sufficient for forming the formula units LiAlO 2 , MgAl 2  O 4 , ZnAl 2  O 4  and AlPO 4 .

This is a continuation, of application Ser. No. 267,266, filed June 29,1972, now abandoned.

BACKGROUND

The invention relates to a transparent glass ceramic containing β-quartzcrystalline solid solutions, which ceramic is colorless and has in thetemperature range from 20° - 300°C a thermal longitudinal expansion ofless than 15 × 10.sup.⁻⁷ /° C.

It is known that fine-grain crystalline solid solutions having anβ-quartz or β-spodumene structure can be precipitated by heat treatmentfrom glasses containing essentially the components SiO₂, Al₂ O₃, P₂ O₅,Li₂ O, MgO and ZnO, if the glasses contain small quantities of so-callednucleating agents as well as the said main constituents. The fine-grainand uniform precipitation of these types of crystals has the effect thatthe formed glass-crystal mixed body, which is generally designated asglass ceramic, is distinguished as regards most properties in acharacteristic manner from the glass from which it was formed.Particularly outstanding is the unusually low thermal expansion of suchbodies, which can even be negative, and which is determined by specialproperties of the crystal structure of the said crystalline solidsolutions. The low thermal expansion leads to an extreme stability withrespect to change in temperature of such glass ceramics, and this is aproperty which is extremely significant for many uses.

Normally, the glass ceramics are opaque or have a porcelain-liketranslucency. Under certain conditions, for example, when the formedcrystals are much smaller than the wavelengths of the visible light, orwhen the refractive indices of the crystals and of the residual glassshow a sufficiently small difference from one another, glass ceramicscan also be transparent. Suitable glass compositions and processes forthe heat treatment in order to produce transparent glass ceramics with avery small thermal expansion are for example indicated in GermanOffenlegungsschrift No. 1 596 860. According to this teaching, theinitial glass has to contain 1.5 to 6% by weight of TiO₂ and 0.5 to 3%by weight of ZrO₂ as nucleus formers, so that a transparent glassceramic can be produced. The content of TiO₂ has the effect, as adispleasing secondary phenomenon, of imparting to the glass ceramic adistinct brown coloring. This is all the more pronounced if, asinstructed in U.S. Pat. No. 3,157,522, only TiO₂ is used as nucleusformer and in fact in an amount of 3 to 7% by weight.

This brown coloring is normally undesirable and reduces the value of theproduct. Accordingly, experiments have been undertaken for developingcolorless glass ceramics with small thermal expansion. One possibilityof obtaining such bodies is described in German Offenlegungsschrift 1496 098. The glass ceramics indicated therein do not contain any TiO₂,but only ZrO₂, as nucleus-forming constituent, and this in an amount of2 to 15% by weight. Since the initial glasses being used contain atleast 60% of SiO₂ and also other predominantly high-meltingconstituents, fusion temperatures of at least 1600°C and sometimes up to1800°C are necessary for the preparation thereof and these temperaturescan only be utilized at very great expense. This circumstance greatlyrestricts the utility of the said glass-crystal mixed bodies, althoughthey have the required freedom from color and in part are satisfactoryas regards their thermal expansion.

THE INVENTION

My own experiments for producing transparent glass ceramics with smallthermal expansion from more easily meltable glasses by addition of ZrO₂as single nucleus-forming constituent were not successful. With smalladditions of TiO₂ as well as ZrO₂, the stated results can be obtained,but then again there is also obtained the aforementioned, undesirablecoloring.

The object of the present invention is to provide glass ceramics withsmall thermal expansion, which are nevertheless transparent andcolorless.

This object is achieved according to the invention by basic glasseshaving a specified composition range as hereinafter more fully outlinedhaving added thereto a mixture of ZrO₂ and Ta₂ O₅ as nucleus-formingconstituent. It was surprisingly found that the nucleus-forming actionof such a mixture in many glasses which are capable of precipitatingcrystalline solid solutions having a β-quartz structure isextraordinarily strong, so that, using a suitable heat treatmentprocess, such a large number of irregularly oriented crystals is formedand with it the average size of said crystals remains so small that aglass ceramic of excellent transparency is obtained. Since both ZrO₂ andTa₂ O₅ are per se colorless and also do not tend to form coloringcomplexes with other constituents, such glass ceramics do not have anyessential color, provided other coloring substances are not deliberatelyadded.

It is possible, but not certain, that the nucleus-forming action of theZrO₂ -Ta₂ O₅ mixture is due to the fact that, during the heat treatment,a large number of minute crystals of the structural type of the cubicZrO₂ are formed, which in their turn serve as starting point for thesubsequent crystallization of the crystalline solid solutions of thestructural type of β-quartz. It was proved by X-ray diffraction that alarge number of such crystals having the structural type of the cubicZrO₂ is present after the conversion process in the glass ceramic andthat their average size reaches at most about 100 Angstroms. The Ta₂ O₅does not occur as an independent crystalline phase, but is in allprobability included in the form of a solid solution into the minutecrystals with the structural type of the cubic ZrO₂.

The content by weight of Ta₂ O₅, for best results, should notsubstantially exceed that of the ZrO₂, since otherwise clouding of theglass ceramic may occur. Best results are obtained if the content of Ta₂O₅ amounts to about two thirds of the ZrO₂ content. It is difficult withusual melting temperatures of at most 1600°C to dissolve more than about7% by weight of ZrO₂ in the glass melts having the composition range asmentioned below. On the other hand, at least about 2% by weight of ZrO₂and 1% by weight of Ta₂ O₅ are necessary in order to guarantee a nucleusformation sufficient for producing a transparent glass ceramic. Hence,the addition in accordance with the invention of the nucleus-formingconstituents ZrO₂ and Ta₂ O₅ covers the range from approximately 2% byweight and 1% by weight, respectively, up to approximately 7% by weightof each. Very favorable results are produced if the ZrO₂ content isbetween 3 and 5% by weight and the Ta₂ O₅ content is between 2 and 5% byweight. The glasses can be and preferably are free of TiO₂.

It is not guaranteed by the use of ZrO₂ and Ta₂ O₅ as nucleus-formingagents that transparent glass ceramics can be produced from all glasseswhich are capable, because of their composition, to precipitate arelatively large quantity of β-quartz crystalline solid solutions. Frommany glasses, even with careful heat treatment, generally only opaque ortranslucent products are formed, of which the thermal expansion ismoreover often considerably high.

It has been found that the production of transparent glass ceramics witha thermal expansion of below 15 × 10.sup.⁻⁷ /°C in the temperature rangefrom 20 to 300°C is only possible if the initial glasses according tothe present invention belong to the composition range which is set outin Table 1. One essential condition in this respect is also that theproportions by weight of Li₂ O, MgO, ZnO and P₂ O₅ have to be such thatthe existing Al₂ O₃ content is approximately just sufficient for formingthe formula units LiAlO₂, MgAl₂ O₄, ZnAl₂ O₄ and AlPO₄ ; the residue orthe deficit of Al₂ O₃ should not exceed approximately 3% by weight, butin any case not exceed 4% by weight. Otherwise, the danger exists ofonly a quite insufficient quantity of β-quartz crystalline solidsolutions being formed or of no transparent glass ceramic beingobtained.

                  Table 1                                                         ______________________________________                                        Composition, Oxide Basis, % by Weight                                                  Broad Range   Preferred Range                                        ______________________________________                                        SiO.sub.2  45 to 68        60 to 68                                           Al.sub.2 O.sub.3                                                                         15 to 30        16 to 23                                           P.sub.2 O.sub.5                                                                           0 to 10        0 to 6                                             Li.sub.2 O 2 to 6          2.5 to 4.5                                         MgO        0 to 3          0 to 2                                             ZnO        0 to 8          0 to 6                                             ZrO.sub.2  2 to 7          3 to 5                                             Ta.sub.2 O.sub.5                                                                         1 to 7          1 to 5                                             ______________________________________                                    

In addition to the constituents mentioned in Table 1, the glass cancontain relatively small amounts of other substances. Hence, and by wayof example, for the purpose of refining the glass, it is frequentlyexpedient to add about 1% of As₂ O₃ or Sb₂ O₃ in association with about1% of Na₂ O or K₂ O, which are introduced as nitrate into the batch. Theinclusion of a few percent, e.g., up to 2% by weight, of BaO or CaO maybe advisable for facilitating the fusibility and does not have to have adeleterious effect on the properties of the glass ceramic according tothe invention. Rare earth oxides can even be included in the batch inamounts up to 12% by weight without causing any harm. It is true thatsuch glasses, i.e., those containing rare earth oxides, have arelatively short processing range, but the glass ceramics which areformed therefrom are characterized by high mechanical strength andexcellent transparency. Naturally, they are only colorless if the rareearth ions which are used have no neutral color.

In individual cases, also other constituents in small quantities can beharmless and can provide advantages for special applications. Generally,however, such additions should be kept small, since otherwise there is agreat danger that it is not possible to produce the properties accordingto the invention. In general, the constituents listed in Table 1 shouldmake up at least 85% by weight of the total composition.

The glasses can be produced by the procedures known for lithium aluminumsilicate glasses. Thus, the glasses are melted at temperatures of, e.g.,at most 1600°, usually at about 1550°, then brought into the requiredform and annealed in a cooling process. The Ta₂ O₅ can be added to thebatch as such, i.e., as the oxide.

The following heat treatment for the conversion into thevitreous-crystalline state can be developed in very many different ways.What is desirable for a temperature control with the conversion is verydependent on the composition of the glass and the shape of the article.Satisfactory results can always be achieved if the article is heated upat about 10°/h to a temperature which is between 820° and 860°, is leftat this temperature for 2 to 6 hours and is then cooled at approximately100°/h. More rapid heating procedures are also frequently possible; inthis case, a residence time of 2 to 4 hours in the temperature rangebetween 680° and 750° is desirable, since in this temperature rangealready transpositions take place in the glass, which make possible therapid formation of a large number of nuclei for the crystallization ofthe β-quartz crystalline solid solutions. The cooling of the articlesafter the conversion or transformation does not present anydifficulties, because of the high resistance to temperature change ofthe glass ceramic.

The following examples of compositions as indicated in Table 2 areintended to explain the subject of the invention, but not to limit it.Set out in Table 3 are property values for the glasses and propertyvalues and production procedures for glass ceramics of Table 2.

                                      TABLE 2                                     __________________________________________________________________________    (% by Weight)                                                                 Example                                                                       No.  SiO.sub.2                                                                        Al.sub.2 O.sub.3                                                                  P.sub.2 O.sub.5                                                                   Li.sub.2 O                                                                        K.sub.2 O                                                                         Na.sub.2 O                                                                        MgO ZnO BaO Y.sub.2 O.sub.3                                                                   La.sub.2 O.sub.3                                                                   As.sub.2 O.sub.3                                                                   ZrO.sub.2                                                                         Ta.sub.2            __________________________________________________________________________                                                              O.sub.5             1    55.0                                                                             24.0                                                                              6.0 3.5 0.5 --  1.0 2.0 --  --  --   1.0  4.0 3.0                 2    64.0                                                                             20.0                                                                              3.0 3.8 0.5 --  1.4 0.8 0.5 --  --   1.0  3.0 2.0                 3    45.0                                                                             27.5                                                                              7.5 5.0 --  --  0.5 1.0 1.5 --  --   --   6.0 6.0                 4    61.4                                                                             21.0                                                                              --  2.8 0.0 --  1.0 6.0 1.6 --  --   0.8  3.5 1.5                 5    65.0                                                                             21.0                                                                              --  3.0 --  --  3.0 1.1 --  --  --   0.9  3.0 3.0                 6    49.5                                                                             18.0                                                                              5.0 4.1 --  --  --  1.4 --  --  8.0  --   7.0 7.0                 7    47.0                                                                             19.0                                                                              5.0 4.5 --  0.5 --  --  1.0 --  10.0 1.0  6.0 6.0                 8    56.5                                                                             20.0                                                                              3.0 4.0 1.0 --  1.0 1.5 --  3.0 --   1.0  5.0 4.0                 __________________________________________________________________________

                                      TABLE 3                                     __________________________________________________________________________               Glass                              Glass Ceramic                   Example                                                                             Tg   α20-300°                                                                 Density                    α20-300°                                                               α20-700°                                                         3     Density             No.   (°C)                                                                        (10.sup.-.sup.7 /°C)                                                           (g/cc.sup.3)                                                                        Heat treatment       (10.sup.7 /°C)                                                               (10.sup.-.sup.7                                                               /°C)                                                                         (g/cc.sup.3)        __________________________________________________________________________    1     687  39      2.515 at 10°/h up to 830°, 3 h holding                                time                 - 7.6 +4.0  2.598               2     692  39            at 10°/h up to 830°, 3 h holding                                time                 - 1.0                           3     690  51      2.647 at 10°/h up to 830°, 3 h holding                                time                 - 5.8       2.673               4     675  36      2.587 100°/h up to 700°, 2 h holding                                  time                                                                          100°/h up to 800°, 2 h holding                                  time                 - 1.7 +6.5  2.651               5     685  39            100°/h up to 700°, 2 h holding                                  time                                                                          100°/h up to 800°, 2 h holding                                  time                 +10.0                           6     677  50      2.790 at 10°/h up to 790°, 5 h holding                                time                 - 3.8 +8.5  2.879               7     675  48      2.804 at 100°/h up to 690°, at                                        10°/h                                                                  up to 870°, 3 h holding                                                                     + 2.9       2.877               8     680  50      2.631 at 50°/h up to 830°, 3 h holding                                time                 + 7.0       2.616               __________________________________________________________________________

If the compositions indicated in the examples are melted without thecomponent Ta₂ O₅, a glass ceramic having satisfactory transparency isnot obtained from any of these glasses, but always translucent products.On the other hand, if the Ta₂ O₅ is replaced by Nb₂ O₅, transparentglass ceramics can be obtained, but these have a brownish coloration.

By the heat treatment in all indicated examples, particularly β-quartzcrystalline solid solutions are formed as well as those crystals alreadymentioned with the structural type of the cubic ZrO₂. The proportion ofthe β-quartz crystalline solid solutions in all cases referred to,amounts to at least 50% and usually even about 70% of the volume of theglass ceramic. Where substantial amounts of components such as CaO, BaOor rare earth oxides are included in the composition, also othercrystalline phases can occur in a subordinate proportion. Theseadditional phases usually cannot be reliably identified by X-rayphotography because of their small quantity and small crystallite size.

If during the heat treatment a maximum temperature which, depending oncomposition, is between 850° and 950°, is exceeded or is maintained toolong, the transparency of the glass ceramic articles is lost, because ofincrease in grain size of the β-quartz crystalline solid solutionsbecause of collective crystallization, or because of recrystallizationto crystals having the β-spodumene structural type. The same limits areapplicable as regards the use of the finished glass ceramics atrelatively high temperatures

The lower limit for the expansion coefficient x can be - 10 × 10.sup.⁻⁷/°C, preferably - 5 × 10.sup.⁻⁷ /°C.

What is claimed is:
 1. Transparent colorless glass ceramic with a linearthermal expansion coefficient less than 15 × 10.sup.⁻⁷ /°C in thetemperature range of about 20° to 300°C, containing irregularly orientedcrystals of the structural type of β-quartz, consisting essentially ofthe following on an oxide basis:

    ______________________________________                                        SiO.sub.2   45 to    68%     by weight                                        Al.sub.2 O.sub.3                                                                          15 to    30%     by weight                                        P.sub.2 O.sub.5                                                                           0 to     10%     by weight                                        Li.sub.2 O  2 to     6%      by weight                                        MgO         0 to     3%      by weight                                        ZnO         0 to     8%      by weight                                        ZrO.sub.2   2 to     7%      by weight                                        Ta.sub.2 O.sub.5                                                                          1 to     7%      by weight                                        ______________________________________                                    

the contents of Li₂ O, MgO, ZnO and P₂ O₅ being such that the existingAl₂ O₃ content, to within a residue or a deficit of Al₂ O₃ of up toabout 3% by weight, is sufficient for forming the formula units LiAlO₂,MgAl₂ O₄ ZnAl₂ O₄ and AlPO₄.
 2. Glass ceramic according to claim 1, atleast 85% by weight of the glass ceramic being of the compositionstated.
 3. Glass ceramic according to claim 1, characterized in that itadditionally contains up to 2% by weight of BaO and/or CaO.
 4. Glassceramic according to claim 1, characterized in that it contains up to12% by weight of rare earth oxides having no natural color.
 5. Glassceramic according to claim 3, characterized in that it contains up to12% by weight of rare earth oxides.
 6. Glass ceramic according to claim1, the ZrO₂ content and Ta₂ O₅ content being:

    ______________________________________                                        ZrO.sub.2         3 to 5% by weight                                           Ta.sub.2 O.sub.5  2 to 5% by weight                                           ______________________________________                                    

with the proviso that the Ta₂ O₅ content is equal to or smaller than theZrO₂ content.
 7. Glass ceramic according to claim 2, the ZrO₂ contentand Ta₂ O₅ content being:

    ______________________________________                                        ZrO.sub.2         3 to 5% by weight                                           Ta.sub.2 O.sub.5  2 to 5% by weight                                           ______________________________________                                    

with the proviso that the Ta₂ O₅ content is equal to or smaller than theZrO₂ content.
 8. Glass ceramic according to claim 3, the ZrO₂ contentand Ta₂ O₅ content being:

    ______________________________________                                        ZrO.sub.2         3 to 5% by weight                                           Ta.sub.2 O.sub.5  2 to 5% by weight                                           ______________________________________                                    

with the proviso that the Ta₂ O₅ content is equal to or smaller than theZrO₂ content.
 9. Glass ceramic according to claim 4, the ZrO₂ contentand Ta₂ O₅ content being:

    ______________________________________                                        ZrO.sub.2         3 to 5% by weight                                           Ta.sub.2 O.sub.5  2 to 5% by weight                                           ______________________________________                                    

with the proviso that the Ta₂ O₅ content is equal to or smaller than theZrO₂ content.
 10. Glass ceramic according to claim 5, the ZrO₂ contentand Ta₂ O₅ content being:

    ______________________________________                                        ZrO.sub.2         3 to 5% by weight                                           Ta.sub.2 O.sub.5  2 to 5% by weight                                           ______________________________________                                    

with the proviso that the Ta₂ O₅ content is equal to or smaller than theZrO₂ content.
 11. Glass ceramic according to claim 1, the Ta₂ O₅ contentbeing about two-thirds of the ZrO₂ content.
 12. Glass ceramic accordingto claim 6, the Ta₂ O₅ content being about two-thirds of the ZrO₂content.
 13. Glass ceramic according to claim 1, free of TiO₂.
 14. Glassconvertible to a transparent colorless glass ceramic, consistingessentially of the following on an oxide basis:

    ______________________________________                                        SiO.sub.2         45 to 68% by weight                                         Al.sub.2 O.sub.3  15 to 30% by weight                                         P.sub.2 O.sub.5    0 to 10% by weight                                         Li.sub.2 O        2 to 6% by weight                                           MgO               0 to 3% by weight                                           ZnO               0 to 8% by weight                                           ZrO.sub.2         2 to 7% by weight                                           Ta.sub.2 O.sub.5  1 to 7% by weight                                           ______________________________________                                    

the contents of Li₂ O, MgO, ZnO and P₂ O₅ being such that the existingAl₂ O₃ content, to within a residue or a deficit of Al₂ O₃ of up toabout 3% by weight, is sufficient for forming the formula unitsLiAlO.sub. 2, MgAl₂ O₄, ZnAl₂ O₄ and AlPO₄.
 15. Glass ceramic accordingto claim 1, consisting essentially of the following on an oxide basis

    ______________________________________                                        SiO.sub.2      60 to 68 % by weight                                           Al.sub.2 O.sub.3                                                                             16 to 23 % by weight                                           P.sub.2 O.sub.5                                                                              0 to 6 % by weight                                             Li.sub.2 O     2.5 to 4.5 by weight                                           MgO            0 to 2 by weight                                               ZnO            0 to 6 by weight                                               ZrO.sub.2      3 to 5 by weight                                               Ta.sub.2 O.sub.5                                                                             1 to 5 by weight.                                              ______________________________________                                    


16. Glass according to claim 1, characterized in that it contains up to12% by weight of La₂ O₃ or Y₂ O₃.
 17. Glass according to claim 3,characterized in that it contains up to 12% by weight of La₂ O₃ or Y₂O₃.
 18. Glass ceramic according to claim 1, and consisting essentiallyof:

    ______________________________________                                        SiO.sub.2      45 to 65 % by weight                                           Al.sub.2 O.sub.3                                                                             18 to 27.5 % by weight                                         P.sub.2 O.sub.5                                                                              0 to 7.5 % by weight                                           Li.sub.2 O     2.8 to 5.0 % by weight                                         K.sub.2 O      0 to 1.0 % by weight                                           Na.sub.2 O     0 to 0.5 % by weight                                           MgO            0 to 3 % by weight                                             ZnO            0 to 6 % by weight                                             BaO            0 to 1.6 % by weight                                           Y.sub.2 O.sub.3                                                                              0 to 3.0 % by weight                                           La.sub.2 O.sub.3                                                                             0 to 10 % by weight                                            As.sub.2 O.sub.3                                                                             0 to 1.0 % by weight                                           ZrO.sub.2      3 to 7 % by weight                                             Ta.sub.2 O.sub.5                                                                             1.5 to 7 % by weight.                                          ______________________________________                                    


19. Glass ceramic according to claim 18, consisting essentially of:

    ______________________________________                                        SiO.sub.2      55.0 % by weight                                               Al.sub.2 O.sub.3                                                                             24.0 % by weight                                               P.sub.2 O.sub.5                                                                              6.0 % by weight                                                Li.sub.2 O     3.5 % by weight                                                K.sub.2 O      0.5 % by weight                                                MgO            1.0 % by weight                                                ZnO            2.0 % by weight                                                As.sub.2 O.sub.3                                                                             1.0 % by weight                                                ZrO.sub.2      4.0 % by weight                                                Ta.sub.2 O.sub.5                                                                             3.0 % by weight.                                               ______________________________________                                    


20. Glass ceramic according to claim 18, consisting essentially of:

    ______________________________________                                        SiO.sub.2      64.0 % by weight                                               Al.sub.2 O.sub.3                                                                             20.0 % by weight                                               P.sub.2 O.sub.5                                                                              3.0 % by weight                                                Li.sub.2 O     3.8 % by weight                                                K.sub.2 O      0.5 % by weight                                                MgO            1.4 % by weight                                                ZnO            0.8 % by weight                                                BaO            0.5 % by weight                                                As.sub.2 O.sub.3                                                                             1.0 % by weight                                                ZrO.sub.2      3.0 % by weight                                                Ta.sub.2 O.sub.5                                                                             2.0 % by weight.                                               ______________________________________                                    


21. Glass ceramic according to claim 18, consisting essentially of:

    ______________________________________                                        SiO.sub.2      45.0 % by weight                                               Al.sub.2 O.sub.3                                                                             27.5 % by weight                                               P.sub.2 O.sub.5                                                                              7.5 % by weight                                                Li.sub.2 O.sub.5                                                                             5.0 % by weight                                                MgO            0.5 % by weight                                                ZnO            1.0 % by weight                                                BaO            1.5 % by weight                                                ZrO.sub.2      6.0 % by weight                                                Ta.sub.2 O.sub.5                                                                             6.0 % by weight.                                               ______________________________________                                    


22. Glass ceramic according to claim 18, consisting essentially of:

    ______________________________________                                        SiO.sub.2      61.4 % by weight                                               Al.sub.2 O.sub.3                                                                             21.0 % by weight                                               Li.sub.2 O     2.8 % by weight                                                K.sub.2 O      0.4 % by weight                                                MgO            1.0 % by weight                                                ZnO            6.0 % by weight                                                BaO            1.6 % by weight                                                As.sub.2 O.sub.3                                                                             0.8 % by weight                                                ZrO.sub.2      3.5 % by weight                                                Ta.sub.2 O.sub.5                                                                             1.5 % by weight.                                               ______________________________________                                    


23. Glass ceramic according to claim 18, consisting essentially of:

    ______________________________________                                        SiO.sub.2      65.0 % by weight                                               Al.sub.2 0.sub.3                                                                             21.0 % by weight                                               Li.sub.2 O     3.0 % by weight                                                MgO            3.0 % by weight                                                ZnO            1.1 % by weight                                                As.sub.2 O.sub.3                                                                             0.9 % by weight                                                ZrO.sub.2      3.0 % by weight                                                Ta.sub.2 O.sub.5                                                                             3.0 % by weight.                                               ______________________________________                                    


24. Glass ceramic according to claim 18, consisting essentially of:

    ______________________________________                                        SiO.sub.2      49.5 % by weight                                               Al.sub.2 O.sub.3                                                                             18.0 % by weight                                               P.sub.2 O.sub.5                                                                              5.0 % by weight                                                Li.sub.2 O     4.1 % by weight                                                ZnO            1.4 % by weight                                                La.sub.2 O.sub.3                                                                             8.0 % by weight                                                ZrO.sub.2      7.0 % by weight                                                Ta.sub.2 O.sub.5                                                                              7.0 % by weight.                                              ______________________________________                                    


25. Glass ceramic according to claim 18, consisting essentially of:

    ______________________________________                                        SiO.sub.2      47.0 % by weight                                               Al.sub.2 O.sub.3                                                                             19.0 % by weight                                               P.sub.2 O.sub.5                                                                              5.0 % by weight                                                Li.sub.2 O     4.5 % by weight                                                Na.sub.2 O     0.5 % by weight                                                BaO            1.0 % by weight                                                La.sub.2 O.sub.3                                                                             10.0 % by weight                                               As.sub.2 O.sub.3                                                                             1.0 % by weight                                                ZrO.sub.2      6.0 % by weight                                                Ta.sub.2 O.sub.5                                                                             6.0 % by weight.                                               ______________________________________                                    


26. Glass ceramic according to claim 18, consisting essentially of:

    ______________________________________                                        SiO.sub.2      56.5 % by weight                                               Al.sub.2 O.sub.3                                                                             20.0 % by weight                                               P.sub.2 O.sub.3                                                                              3.0 % by weight                                                Li.sub.2 O     4.0 % by weight                                                K.sub.2 O      1.0 % by weight                                                MgO            1.0 % by weight                                                ZnO            1.5 % by weight                                                Y.sub.2 O.sub.3                                                                              3.0 % by weight                                                As.sub.2 O.sub.3                                                                             1.0 % by weight                                                ZrO.sub.2      5.0 % by weight                                                Ta.sub.2 O.sub.5                                                                             4.0 % by weight.                                               ______________________________________                                    


27. Glass ceramic according to claim 1, the residue or excess of Al₂ O₃being +2.7 to -1.34.